Capillary electrophoresis is a technique for analyzing and/or purifying a wide variety of biochemical substances or analytes such as proteins, nucleic acids, carbohydrates, hormones and vitamins. The basic capillary electrophoretic apparatus consists of a capillary column having the ends thereof positioned in reservoirs containing electrodes. A conducting liquid or buffer solution disposed in the reservoirs and the capillary column comprises the electrophoretic conductive circuit. The analyte is injected into the appropriate end of the capillary column and a voltage applied across the electrodes. The applied voltage causes the analyte to migrate electrophoretically through the capillary column past a prepositioned on-column detection device to generate an electropherogram, a graphical representation of the analyte.
The capillary columns utilized in electrophoretic systems are thin walled, hollow tubes preferably formed from a low specific heat, non-electrically conducting material such as fused silica. Capillary columns for use in electrophoretic systems typically have a length in the range of 10 to 100 cm, an internal diameter in the range of 25 to 200 microns, and an outer diameter in the range of 125 to 350 microns, depending upon the I.D. of the capillary column.
A quartz based fused silica capillary column having the above-disclosed dimensions, however, is relatively fragile and susceptible to fracture or breakage due to handling and/or externally applied forces. Therefore, the mechanical strength and flexibility of capillary columns for electrophoretic systems are generally enhanced by applying an external protective coating of a polymer such as polyimide to the capillary columns.
The polymer coating, however, would interfere with the operation of the on-column detection device inasmuch as the polymer coating inhibits the measurement or sensing of the electromigrating analyte during passage through the interior of the capillary column. Therefore, as illustrated in FIG. 1, the polymer coating of a capillary column CC must be modified to include detection "windows W". It is to be understood that "windows" is used in a generic sense to include not only a window W formed by selectively removing a 360 degree band of polymer coating as shown in FIG. 1 but also the selective removal of the polymer coating to form partial windows. Such modification includes the selective removal of the polymer coating at a predetermined position on the capillary column.
While the Background has been set forth in terms of capillary columns used in electrophoretic apparatus, it will be appreciated that the problems described in the preceding paragraphs are inherent in other applications such as open-tube liquid chromatography and supercritical fluid chromatography. There are many applications wherein sensing/measurement or other type windows must be formed in small-diameter tubes or columns which are mechanically reinforced by means of an external polymer-type coating by selective removal of the polymer-type coating. It is therefore to be understood that the window burner of the present invention is not limited to polymer-coated capillary columns utilized in electrophoretic apparatus.
Several methods are known for removing the polymer coating on quartz based fused silica capillary columns. A razor blade may be used to scrape the polymer coating from the capillary column. This method is disadvantageous in that the column is extremely susceptible to fracture or breakage. The slightest etching of the glass during removal of the polymer coating reduces the mechanical strength of the column. In addition, fracture or breakage may be incurred due to the increased handling or the force applied through the razor blade. In addition, this method is extremely unreliable in producing windows of consistent dimension.
In addition to retention of mechanical integrity, very precise uniformity of dimension must be maintained among windows produced by removing the polymer coating from the capillary columns. This precise uniformity is not attainable through the use of razor blades.
Another method involves the application of an open flame to burn the polymer coating from the capillary column. This method, however, has several drawbacks. A flame device requires a gas supply and a very small needle tip to develop an appropriate flame. Setting up flame devices for operation is a time consuming and labor intensive operation. In addition, the flame and/or gas supply are potential sources of fire or explosion. Flame devices typically generate more heat than is required to remove the polymer coating, oft times resulting in bent, melted or useless capillary columns. Flame devices generally do not provide precise uniformity of dimension in the windows formed, and many such devices create windows so large that the column becomes very fragile.
High voltage arc devices can be used to create windows in capillary columns. These devices, however, do not provide precise uniformity of dimension in the windows created due to the difficulty in controlling the arc generated. In addition, high voltage arc devices tend to be prohibitively expensive for the task, costing several thousands of dollars apiece.